Never seen that issue before - if there's a problem with the preamp, the display usually shows a series of black boxes. If you use the up/down/left/right selection button and channel buttons, does the sound change?
Unfortunately, it sounds like you would need to bring it to an authorize service center for evaluation.

I do agree with you - I'm one who doesn't like using any true stereo sound in a live situation - even if its two cabinets (or separate amps) on either side of the stage, as inevitably, someone in the audience is going to be in a position where they only hear one of the sides well.

Well, that is exactly the point - to get you to buy the Line 6 cabinet! Also of note - the speakers in L6 amps/cabinets are picked to produce the sound of the amplifier/preamp without imparting any real 'color' to the sound (if you doubt this, use the MP3 input to play some music through any L6 amp - they try the same through a non-modeling amp and speaker system).

Yes, you do not want to run just one output from your Spider amp, as that will imbalance the amp and eventually cause a problem. The amp wants to see an 8 ohm (or higher impedance) on each output. Your speaker cabinet is 8 ohms. You'd have to open it up, rewire it with two input jacks, and probably use a couple of power resistors to get the impedance correct.

have not used this amp or its downloads in a while general use is clean sound in which I use Mesa,but have start a new venture were I need a varity of different sounds so here I am need to be retrained FOR LINE 6 USE!

Not sure if you can calibrate that part of the pedal - the pedal does not have a tuner itself, its is just reading the signal from the amp, so it's the signal calibration that is off at the tuner.
You could try reflashing the pedal software (if you dare - sometimes people have had more issues when doing that!) - but you'll need Monkey to recognize the devices first. I had that problem with an old Win machine, eventually used a different computer to connect and get the software updates.

Even though there was no actual input, its possible that connecting the amp this way created an electric feedback circuit and burned out components in the input section of the preamp. The 15 is not a serviceable amp - its very low cost so just an evaluation would cost more than replacing the amp.

You assign the sound patch to the A B C or D location on the amp, and then switch to each with the FBV's buttons. Use the 'save' button to access 'save guitar tone'
Most people don't find the L6 manuals very good - you can find plenty of youtube tutorials with a search.